The present invention relates generally to methods and systems for preparing ultra pure cuprous chloride (CuCl). More particularly, the present invention relates to an improved method and system for preparing CuCl by the reduction of cupric chloride (CuCl.sub.2) to cuprous chloride in the presence of copper metal.
Ultra pure CuCl crystals which are free of cupric ion (Cu++) are useful as an optically nonlinear material that has wide applications for use in electro optic and integrated optic systems. In order for the CuCl crystals to be suitable for such applications, it is essential that the CuCl be free of cupric ions. Due to size and charge disparities, cupric ions cannot substitute for cuprous ions (Cu+) in the cuprous chloride crystal lattice. Unless there are present other impurity species that are size and/or charge-compensating, the cupric ions can only occupy interstitial sites. As a result, the inclusion of cupric ions in the cuprous chloride crystal lattice produces cuprous ion vacancies and/or chloride ion interstices. Cuprous ion vacancies are by far more prevalent. When oxygen ion impurities are also present in the crystal lattice, the oxygen ions may substitute for chloride ions to coexist with either substitutional or interstitial cupric ions. In this fashion, the oxygen ions provide stabilization of the cupric ion within the cuprous chloride crystal lattice. This makes it even more difficult to rid the cuprous chloride crystal or powder of the undesirable cupric ions.
The conventional method of preparing cuprous chloride powder involves immersing copper metal in an aqueous solution of cupric chloride that is highly acidified with hydrochloric acid and blanketed by an inert gas such as nitrogen. The reaction which takes place is: ##STR1## This conventional method is based on a heterogeneous reaction, i.e., one involving reactants in the solid state (copper metal) and liquid solution (CuCl.sub.2). The rate at which the process occurs is limited in its early stages by the magnitude of the liquid-solid interfacial area. Further, as the concentration of reactants in the liquid solution diminishes, the reaction rate decelerates so that the reaction cannot be completed within time intervals of practical duration. In order to speed up the reaction, copper metal in a high surface-to-volume form, such as powder or turnings is used and the reaction mixture is heated to below boiling temperature. This provides increased initial reaction rates, but even then, the reaction rate slows considerably near the end point. As a result, the end point of the reaction cannot be reached within realistic time intervals.
Cupric chloride solution is dark green in color and turns very dark brown upon contact with the copper powder or turnings. The brown coloration gets lighter with the progress of the reaction. The solution becomes colorless when the reaction is complete. However, as mentioned above, the end point of the reaction cannot be reached within a realistic time interval using stoichiometric amounts of copper metal and cupric chloride solution, so that a small amount of cupric ions remain in the solution. Stoichiometric, as used herein, means that when the reaction is complete there are no leftover amounts of either reactant.
Cuprous chloride is very soluble in hydrochloric acid, but only slightly soluble in water. A highly HCL-acidified aqueous medium is used to prevent the premature precipitation of the curpous chloride. Precipitation of cuprous chloride during the reaction is undesirable, since the cuprous chloride precipitates on the surface of the copper metal and hinders the progress of the reaction.
In conventional processes, when the solution is as near to its end point as patience and economics will allow, any excess copper metal is retrieved and the solution is diluted with deoxygenated water in order to reduce its acidity and bring about the precipitation of the cuprous chloride. The precipitate is then filtered off, washed down sparingly with chilled deoxygenated water and vacuum dried. This entire series of steps must be carried out under a blanket of inert gas. Since it is difficult to retrieve any excess copper which may be left in the solution, the reaction is conventionally carried out with an excess of cupric chloride. The excess cupric chloride insures complete copper metal consumption and stays in solution when the solution is diluted and separated from the cuprous chloride precipitate. However, the excess cupric chloride in the solution is undesirable because it provides a possible source of cupric ions to contaminate the cuprous chloride powder.
Cuprous chloride powder prepared according to the above conventional process is available in the highpurity materials market as a powder that is five nines pure (99.999%) on a "metal basis". However, this manner of specification does not regard cupric ions as an impurity in cuprous chloride. As a result, the cuprous chloride powder may include varying amounts of cupric chloride. This explains why the market-variety cuprous chloride comes in the form of a powder that is various shades of green. Cupric ion-free cuprous chloride powder should be white.
It would be desirable to provide a method and system for preparing white cuprous chloride powder which is essentially free of cupric ion and which can be used in the preparation of ultra pure cuprous chloride crystals for use in electro optic and integrated optic system. Further, it would be desirable to provide a method in which the reaction time for the complete conversion of cupric ions to cuprous ions is reduced.